WL#4067: MySQL plugin interface: syntax extension

Plugin should be able to add new SQL syntax that MySQL understands.
In particular it should include:
1. optional clauses in CREATE TABLE (done as WL#2938)
2. optional clauses in SHOW ENGINE xxx
3. possibly a new class of statements ENGINE xxx whatever...
e.g.
ENGINE InnoDB FLUSH LOGS
ENGINE NDB SHOW NODES
ENGINE NDB RESTART NODE

4. anything else ?

with recursive-descent parser one can, technically, extend run-time any branch
of the parse tree